Transparent electrode imaging improvement

ABSTRACT

APPARATUS FOR IMPROVED IMAGING AND FOR ELIMINATING CORONA ARCING IN AN ELECTROPHORETIC IMAGING SYSTEM EMPLOYING A TRANSPARENT ELECTRODE HAVING ELECTRICALLY CONDUCTIVE TRANSPARENT PORTIONS CAUSING VARIATIONS IN THE SURFACE POTENTIAL OF THE ELECTRODE UNDER THE INFLUENCE OF AN ELECTRICAL FIELD. THE ELECTRODE FUNCTIONS WITHIN AN IMAGING   SYSTEM ALSO HAVING A MEANS TO ILLUMINATE THE CONDUCTIVE PORTIONS FOR FORMING AN IMAGE IN A CONFINED ELECTRICAL FIELD IN A MANNER PREVENTING CORONA ARCING BETWEEN ELECTRODES OF THE IMAGING SYSTEM.

Dec. 28, 1971 w. GUNDLACH TRANSPARENT ELECTRODE IMAGING IMPROVEMENT 2Sheets-Sheet 1 Filed May 2, 1969 R O T N E V m ROBERT W. GUNDLACHATTORNEY Dc; 28, 1971 R. w. GUNDLACH 3,630,884

TRANSPARENT ELECTRODE IMAGING IMPROVEMENT Filed May 2. .1969 2SheetsSheet z United States Patent Oifice 3,630,884 Patented Dec. 28,1971 3,630,884 TRANSPARENT ELECTRODE IMAGING IMPROVEMENT Robert W.Gundlach, Victor, N .Y., assignor to Xerox Corporation, Rochester, N.Y.Filed May 2, 1969, Ser. No. 821,369 Int. Cl. B01k 5/00 U.S. Cl. 204300 7Claims ABSTRACT OF THE DISCLOSURE Apparatus for improved imaging and foreliminating corona arcing in an electrophoretic imaging system employinga transparent electrode having electrically conductive transparentportions causing variations in the surface potential of the electrodeunder the influence of an electrical field. The electrode functionswithin an imaging system also having a means to illuminate theconductive portions for forming an image in a confined electrical fieldin a manner preventing corona arcing between electrodes of the imagingsystem.

This invention relates in general to imaging systems and morespecifically to an improved electrophoretic imaging system.

The system improved by this invention is of the type usingphotosensitive radiant energy absorbing particles believed to bear acharge when suspended in a non-conductive liquid carrier and disposed inan electroded system to be exposed to an image radiation configuration.For a detailed description of the operation of this system see Pats.Nos. 3,384,565, issued to V. Tulagin and L. M. Carreira, 3,384,566 to H.E. Clark, and 3,383,993 to S. Yeh, issued on May 21, 1968. The particlesof the system migrate in image configuration providing a visual image atone or both of the electrodes between which they are placed. The systememploys particles which are photosensitive and which apparently undergoa net charge alteration upon exposure to activating radiation byinteraction with one of the electrodes. Various mixtures of two or moredifferent colored particles can be used to secure various colors ofimages and imaging mixes having different spectral response. Thesecolors can be used independently or in subtractive color synthesis. In amonochromatic system the particles will migrate if energy of anywavelength within the panchromatic spectrum of the particle responsestrikes the particle.

It has been found that images produced by the system broadly describedabove may on occasion exhibit uneven density or contrast. Further, theapparatus employed for imaging may be damaged during imaging. It isthought that these difiiculties are caused by varying corona dischargeor air ionization, referred to hereinafter as corona arcing, between theelectrodes used for imaging within the system as one electrodeapproaches in proximity to the other electrode. The invention herein wasdeveloped to eliminate this electric arcing between electrodes whileimproving sensitivity levels of the imaging systems in which it is used.The invention enables a system in which it is used the capability forincreasing field strength during imaging at the most effective positionfor imaging without the consequence of increased corona dischargebetween the electrodes of the system.

Therefore, an object of this invention is to improve electrophoreticimaging systems by eliminating corona arcing between electrodes. Anotherobject of this invention is to improve systems using transparentelectrodes. Still another object is to improve transparent electrodescapable of eliminating corona arcing in imaging systems.

The foregoing objects and others are accomplished in accordance withthis invention by providing a transparent electrode for contact withanother electrode within an electrophoretic imaging system wherein thetransparent electrode has a variation of electrical conductivity acrossits surface. Corona arcing is prevented because the field is at leastreduced in areas where arcing is likely to occur. Also corona arcing isprevented because very high local fields can be reached (around the lineor point conductors) without requiring the usual high voltage across theentire gap. In one embodiment a commutating means contacts a portion ofthe conductive surface to provide a potential thereto. It may be thatother systems exist or will be discovered or invented that requireimprovements similar to those described herein and this invention can beused thereon to improve such a system and such use is contemplatedhereby.

The advantages of this improved transparent electrode andelectrophoretic imaging system will become further apparent uponconsideration of the following detailed disclosure of the invention;especially when taken in conjunction with accompanying drawings;wherein:

FIG. 1 is a side view schematic of an electrophoretic imaging systememploying one embodiment of this invention;

FIG. 2 is a sectional side view schematic illustration of anotherembodiment;

FIG. 3 is a perspective view of a transparent electrode;

FIG. 4 is a sectional side view of a portion of a modified embodiment ofthis invention.

Referring to the drawings, shown in FIG. 1 is a photoelectrophoreticimaging system having an injecting electrode 10 made in accordance withthis invention. It is composed of a transparent ground or etched glasssub strate 12 and an overcoated, transparent electrical conducting layer14. The overcoated conductive layer 14 injecting electrode may be alayer of tin oxide coating or any other coating which is bothtransparent and electrically conducting.

Depovsited on the electrical conducting layer 14 of the injectingelectrode 10 is a thin layer of finely divided photo sensitive particlesdispersed in an insulating liquid carrier. This is the imagingsuspension 16 from which an image is formed.

Adjacent to the electrode 10 and the suspension 16 coated thereon is animaging electrode generally referred to by the numeral 18. The imagingelectrode has a layer 20 of blocking material, that is, material whichonce contacted by photosensitive particles will not inject a sufficientcharge into them to cause them to migrate from the imaging electrode.The inner roller 21 is electrically conductive.

The term photosensitive for the purposes of this invention refers to theproperties of a particle which, once attracted toward the injectingelectrode, will reverse its 3 polarity of charge and migrate away fromit under the influence of an applied electric field when exposed toactivating electromagnetic radiation. The term suspension may be definedas a system having solid particles dispersed in a solid, liquid or gas.Nevertheless, the suspension described in the embodiments herein is ofthe general type including those having a solid suspended in a liquidcarrier. The term injecting electrode refers to the electrode theproperties of which apparently inject charges into photosensitiveparticles activated by electromagnetic radiation while under theinfluence of an electric field.

The photosensitive suspension 16 is subjected to electromagneticradiation in image configuration by, for example, shining a light source22 through an object such as a transparency 25 which is imaged throughlens 26 to the photosensitive suspension 16 on the upper surface of theinjecting electrode 10. More or less simultaneously with the projectionof the object 24 to the suspension 16, an electric field is appliedbetween the electrodes and 18. The imaging electrode is given shape bythe conductive support member 21 and the shaft 23 which is suitablyjournaled for rotation over the injecting electrode 10. Shownschematically in FIG. 1 is an electrical energy source 28 connectedthrough a switch 30 to the conductive roller 21 of the imaging electrode28. The mechanism functions such that when the imaging electrode 18traverses the surface of the injecting electrode 10, the switch 30 isclosed forming an electric field at the interface of the two electrodes.The surface 20 of imaging electrode 18 is preferably negative relativeto the surface 14 of the injecting electrode 10.

The particles within the suspension are non-conductive unless they arestruck with activating radiation. Under the influence of the appliedelectric field, as show in this embodiment, the negative particles comeinto contact with or are closely adjacent to the injecting electrode 10and remain there. When activating radiation strikes the photosensitiveparticles, it makes the particles conductive creating hole-electronpairs of charge carriers which may be considered mobile in nature. Thesenewly created holeelectron pairs within the particles are thought toremain separated before they can re-combine due to the electrical fieldsurrounding the particles between the two electrodes. The negativecharge carriers of these hole-electron pairs move toward the positiveelectrode 10 while the positive charge carriers move toward the negativeimaging elec trode 18. The negative charge carriers near theparticleelectrode interface at electrode 10 can move across the veryshort distance between the particles and the surface 14 leaving theparticles with a net positive charge after sufiicient charge transfer.These net positively charged particles are now repelled away from thepositive surface of the electrode 10 and are attracted toward thenegative imaging electrode 18. The particles struck by activatingradiation of a wavelength to which they are sensitive, i.e., one whichwill cause the formation of hole-electron pairs within the particles,move away from the electrode 10 to the electrode 18 leaving behind onlyparticles which are not exposed to sufiicient electromagnetic radiationin their responsive range to undergo this change. Those particlesremaining function to form the image sought by the imaging system.

The conductive surface 14 of the injecting electrode 10 is shaped, inone embodiment, to have various peaks and troughs as shown in the crosssection of FIG. 1. The peaks and troughs might actually be formed in thesubstrate 12 and coated evenly by the conducting surface 14. On theother hand, it would also be possible to form the peaks and troughs withthe transparent conductive layer 14 alone overlayed on a fiat surface.By shaping the surface of the injecting electrode as done by thisinvention, the field strength of the imaging system at its mostimportant area, that which is between the electrodes, may be increasedwithout deteriorating effects on the image due to corona arcing betweenthe electrodes. The transparent conductive surface may be formed to havepoints, lines or ridges either physically in its surface or in itsconductivity pattern.

What occurs during the imaging of the photoelectrophoretic suspension 16is that the lines of force, and therefore the path of charged particles,will converge to the peaks of the conductive injecting electrode surface14. This is schematically shown by the lines of force 24 shown betweenthe two electrodes of FIG. 1. Therefore, at the suspension-injectingelectrode interface the particles migrate toward the protrusions of theinjecting electrode surface where the lines of force of the field arethe greatest and they exchange charge thereat before reversing theirdirection to migrate to the imaging electrode surface 20. By peaking thefield force at the protrusions of the injecting electrode 10 and havingthe field across the surface 20 of the imaging electrode 18, theprobability of having corona arcing between the two is lessened. Thereason for this is that one may use a lesser field to achieve the sameimaging results due to the shaping of the lines of force, oralternatively one may use the same electrical field as with prior artphotoelectrophoretic imaging systems and due to the shaping of the forcefield the force will be gathered more in the areas where the suspensionis located then in those areas encompassing the air gap between the twoelectrodes.

FIG. 2 shows an alternative embodiment of the invention herein animaging system adapted for continuous imaging of photoelectrophoreticpigments. Here, the injecting electrode 10 is formed as a cylinder witha substrate 32 having conductive portions 34 on its outer surface. Theouter surface of the injecting electrode 10 may be smooth, having areasof varying conductivity along the surface without physical peaks andtroughs therein. The peaks and troughs are of electrical conductivity.This is beneficial to facilitate making uniform ink layers for imagingand to prevent the suspension from settling into the troughs between theridges of conductive material. This naturally will make image transferand cleaning of the injecting electrode surface easier physicaloperations. To this end, an insulating resin could be doctored into thedepression between the peaks of conductive material and left there.Instead of forming the conductive layer of the insulating transparentsurface by attaching or bonding conductive or semi-conductive wire orstrips to the surface, a conductive pattern of chromium, copper,aluminum, or any other suitable conductor or semi-conductor may beevaporated through a screen or grid. For apparatus such as schematicallyshown in FIG. 1, it would be desirable to have a screen or grid pattern,either physical or electrical, that interconnects the various points orridges so that the entire injecting electrode can be grounded orsuitably electrically connected to permit the proper field for imagingacross its entire surface during the contact with the imaging electrode18.

The imaging electrode 18 has a conductive inner core 36 and a blockingouter surface 38. Imaging suspension 16 is deposited on one of theelectrodes, here shown to be the imaging electrode 18, by dispensingthrough suitable means from a reservoir such as held in container 40.

In order to form the images, a stationary mirror 41 is located withinthe rotary injecting electrode 10 to receive the image projected from atransparent object 42 and to direct the image through an exposure slit43 onto the surface of the suspension at the area of contact between thetwo electrodes. The transparency 42 is shown as a film roll or web andis moved synchronously with the rotating injecting and blockingelectrode cylinders to provide a flowing image. The image is projectedthrough a lens 44 for imaging at the intersection of the two electrodesat the slit 45. The image is preferably projected in a plane normal tothe surface of the drums so that distortion of the image plane is heldto minimum. The injecting electrode 10 is grounded by some suitableconnection between the conducting surface ridges or points and ground.The blocking electrode is connected to a potential source 46 such that afield exists across the suspension between the two electrodes with theblocking electrode being shown as positive. It is optically beneficialto have materials for the conductive strips 34 and the substrate 32 ofoptically similar properties.

As the two electrodes rotate in the direction shown by the arrows, aflowing image is formed in the nip between them. A portion of thesuspension 16 will form a transferable suspension image 47 which remainson the injecting electrode while the non-image portion 48 of thesuspension is carried away on the imaging electrode surface, Thedeveloped image is carried on the injecting electrode 10 for contactwith a transfer material 49 which may be an adhesive web or othersuitable transfer material. The web 49 moves along a predetermined paththrough pinch rollers 50 and 51 and around a transfer roller 52. At thetransfer roller 52, the transfer material 49 is pressed into contactwith the injecting electrode 10 in order to pick off the image portion47 of the imaging suspension. The web is then passed through a fixingunit 53 which securely fixes the image 47 to the web 49.

As the electrodes continue to rotate the portion of the suspension 48which does not form part of the image but remains on the imagingelectrode 18 is carried to a cleaning station where suitable means suchas schematically represented by a brush 54 removes the suspension fromthe imaging electrode surface. The cycle may now continue to rotate withother images being formed in the same manner as that describedhereinabove.

FIG. 3 shows a perspective view of an injecting electrode suitable foruse in the apparatus of FIG. 2. However, the transparent conductingstrips of this injecting electrode are not interconnected with oneanother and therefore must be electrically contacted individually inorder to enable field to exist between it and the imaging electrode atthe interface of the two of them. The transparent substrate 56 of thisinjecting electrode is etched or formed in some manner to have fineclosely spaced grooves therein to accept transparent conductive layers57 therein. The outer surface of the insulating substrate 56 and theconductive layers 57 is polished to be completely smooth, The injectingelectrode 10 has alternating conductive and insulating strips across itslength. In this figure, as in all the others discussed, the dimensionsare not to scale. The transparent injecting electrode surface willgenerally include many points or grooves per unit area depending forspacing on the particular method of deposition or screen patternemployed to form the conductive surface on a transparent substrate.

This electrode could be used in the embodiment of FIG. 2 by mounting itover a shaft 58 which would stably and immovably hold the reflectingmirror 41. A suitable frame would mount the injecting electrode 10 forrotation about the shaft without affecting the movement of the mirror orthe connecting arm 59 which is grounded and connects a portion of theconducting in jecting electrode surface to ground as it rotates past theconnecting arm 59. The arm 59 thus acts as an electrical brush to makecontact with a preselected portion of the conductive layers 57. The arm59 is located at the same position as the slit 45 at the interface ofthe two electrodes. Therefore, in this embodiment the field exists in alimited area between the blocking electrode surface and a limitedportion of the injecting electrode conductive layers 57 connected toground by the arm 59.

By so limiting the effect of the field between the two electrodes thisstructure enables more efficient field utilization as described in theprevious figures and further reduces the problem of corona dischargeacross the air gap between the two electrodes by eliminating the fieldexisting across the air gap of the two electrodes. This is so becausethe connecting arm 59 is limited in con- 6 tact to the portion ofconductive strips 57 that is at or near the interface of the twoelectrodes and covered by imaging suspension during the imaging process.

FIG, 4 schematically represents a modified embodiment of an injectingelectrode 10 for use in the photoelectrophoretic imaging system. Inconjunction with the conductivity pattern disclosed for the injectingelectrodes described above, optical sensitivity of the process isincreased by converging the light from the projected image to the sameminute areas on the surface of the injecting electrode at which theelectrical field is the greatest. Those areas, of course, being thepeaks or ridges of conductive material on the surface of the injectingelectrode 10. An example of increased optical efiiciency in conjunctionwith the increased electrical field response efliciency is shown in FIG.4 where a lenticulated surface 60 is formed into the injecting electrodeon the side opposite the conductive surface used to contact thesuspension. In this embodiment the light rays 62 eminating from theprojected object strike the lenticular surface 60 of the injectingelectrode and are converged to the conducting portion 64 of theinjecting electrode. The lenticular surface can be a cylindrical lenssystem or any other light gathering means that can selectively gatherlight to the particular locations of the conductive depositions. In thisembodiment, the conducting portions 64 are placed on the injectingelectrode substrate 66 and an insulating solid material 68 is placed inthe trough between the conductive portions of the injecting electrode.This provides a smooth surface for contact with the imaging suspension.

Various values can be used within the system to form images according tothe known photoeleotrophoretic process and reference is directed to thepatents cited above for the various parameters of the process and thematerials used therein. Various materials for forming conductive,semi-conductive and insulating areas within the injecting electrode canbe used provided they function according to the requirements hereinspecified.

While this invention has been described with reference to the structuresdisclosed herein, it is not confined to the details set forth since theyare merely illustrative of the invention; and this application isintended to cover such modifications or changes as may come within thepurposes of the improvements or scope of the following claims.

What is claimed is:

1. In an apparatus for imaging electrophoretic suspensions having afirst transparent electrode adapted to support an image formed from thesuspension,

a second electrode for contacting the suspension in proximity to thefirst electrode such that the suspension is maintained therebetween in acontact zone,

means to couple the electrodes to an electrical potential supply source,and

means to expose the suspension between the electrodes to activatingelectromagnetic radiation, the improvement in said transparent electrodeincluding,

a surface of varying electrical conductivity having a plurality ofvariations within the area equivalent to the contact zone said surfacebeing intimately attached to the transparent electrode.

2. The apparatus of claim 1 wherein said surface includes a roughtransparent electrically insulating base and a continuous, transparent,electrically conductive material intimately bonded thereto.

3. The apparatus of claim 1 wherein said surface includes a smoothtransparent electrically insulating base and a discontinuoustransparent, electrically conductive material intimately bonded thereto.

4. The apparatus of claim 1 further including a lenticular film on saidtransparent electrode on the side opposite said surface, said filmpositioned to be capable of gathering light striking it and refractingsuch light toward the higher conductive portion of said electricallyconductively varying surface.

5. The apparatus of claim 1 wherein said surface comcal field betweensaid isolated portion and said elecprises peaks and troughs oftransparent, electrically controde. ductive material. References Cited6. The apparatus of claim 5 wherein said troughs in- UNITED STATESPATENTS clude relatively electrically insulating material overcoatingthe electrically conductive material therein such that 5 Egg??? et thesurface for contact with the suspension is smooth. 1 1c 7. The apparatusof claim 5 further including, JO H MACK p i E i means to electricallyisolate a portion of the transparent electrode in the Contact Zone and mR. L. ANDREWS, Assistant Examiner means to electrically activate saidisolated portion of U,S CL X R,

said transparent electrode to limit the applied electri- 20 4 181

